I recently started working at ABB as a engineer. I wish someone here could help with some questions: 1) In a single phase IT system, why does the circuit breakers have to be bipolar? I believe it is because of a second fault occurence, but i don't really understand what happens. 2) If you connect a switchgear to the same earthing system as the transformer neutral is connected, instead of having a cable to directly perform this connection, does it makes your system a TT system? Or the fact that the switchgear is in the same earthing system as the source neutral makes it a TN system? 3) In a three phase system whose neutral is grounded through a resistor, why is it not recommended the existence of single phase loads? 4) Is it necessary to place a CB before and after a potential transformer to guarantee its protection? Does these CB need to have any special features? If anyone knows any articles or papers about these subjects I'd appreciate!